The invention relates to a process for controlling the reactive deposit of a coating on a substrate with a magnetron cathode operated at constant power and a target having an electrically conductive component of the coating material. The process includes measuring the intensity of at least one spectral line of the target material in the plasma of a sputtering process, and admitting reaction gas in the vicinity of the target according to this intensity. PROCESS AND APPARATUS FOR CONTROLLING THE REACTIVE DEPOSIT OF COATINGS ON SUBSTRATES BY MEANS OF MAGNETRON CATHODES
The invention relates to a controlling process according to the general part of claim 1.
In such coating processes, an electrically conductive target, usually a metal target, is sputtered in a reactive atmosphere so that the magnetron cathode can be supplied with direct current. A coating of the reaction product of the target material then deposits itself on the substrate, and the composition of the coating material depends very greatly on the atmosphere surrounding the magnetron cathode.
In this kind of coating process, three different modes of operation of the magnetron cathode are distinguished, namely the "metallic mode," "reactive mode," and "transitional mode." In the "metallic mode" the sputtering surface of the target is of a metallic, i.e., conductive character. In the "reactive mode" a corresponding reaction product is found on the sputtering surface of the target, and, as a rule, it is an electrical insulator, and the sputtering rate, i.e., the amount of target material sputtered per unit time, is considerably reduced. The "transitional mode" is between these two operating modes and is considered to be unstable to a very great extent, because it threatens to tilt in the one or in the other direction. This "transitional mode" is a peculiarity typical of magnetron cathodes, in which an erosion pit forms due to the locally restricted sputtering process, and becomes increasingly deeper. While the walls of the erosion pit can still be kept very largely free of reaction products, the rest of the target surface becomes coated with the reaction products, and thus produces the unstable operating behavior previously described.
In order to prevent or suppress this unstable operation insofar as possible, it is already known to feed the reaction gas in at a point that is remote from the target surface and shielded by a mask, and to feed into the vicinity of the target an inert gas which produces the sputtering (DE-OS 33 31 707). If, when employing such a measure, the feeding of the reaction gas is performed according to the intensity of one or more spectral lines, the result would be time constants that are extraordinarily long for such a control method, and which are unacceptable in a production process, especially a continuous production process. Moreover, in such a case a slow alternation takes place between the "metallic mode" and the "reactive mode" especially because the breakdown of the reaction product on the sputtering surface of the target takes, relatively, a very long time on account of the sputtering rate which in this case is very low.
It has already been recognized that this alternation between the "metallic mode" and the "reactive mode" can be expressed by a hysteresis curve if the reaction gas flow exceeding the stoichiometric ratio is plotted over the reaction gas flow (EP-OS 0 121 019), to which U.S. Pat. No. 4,428,811 corresponds. Even a control process founded on this relationship and using a mass spectrometer does not lead to the elimination of the instability problems, because the time constants of the individual components of the measuring and control circuit are too long.
In reactive coating processes the object is, as a rule, to produce a chemical compound having the required coating properties. These coating properties are to be produced within close tolerances and with great long term stability. A whole series of proposals have been made for regulating a parameter of the process of magnetron sputtering by means of control circuits having a sensor for detecting a particular coating property. All proposals, however, have led to only very limited success. This is because many coating properties cannot be measured in the coating zone, or they can be measured only very imprecisely due to the effect of the plasma on the measuring signals. In such control circuits, therefore, the measurement has always been performed outside of the coating zone. An obstacle to the achievement of close tolerances in this kind of procedure is the time difference between the end of the coating process and the measurement. Since the time constant has to be made substantially greater than this time difference, in the interest of the stability of the control circuit, the result is large substrate areas which fail to be coated within the tolerances. These substrate areas can extend over several of the substrates or, in the case of film coating, over great lengths of the film, which consequently have to be considered as rejects.
Still more important than the disadvantages cited above of using the measured value of a property of the coating for control purposes is the disadvantage that such control circuits do not guarantee the stability of the reactive coating process. Causes of such instabilities are, for example, undesired changes in the gas composition, and arc discharges at the target which express themselves in variations in the properties of the deposited coatings. Such instabilities can tilt the process away from the "metallic mode" to the "reactive mode." Such a tilt during the coating of the substrate signifies complete rejection as far as the coating properties are concerned.
This so-called "tilting" occurs at a critical flow of the reaction gas, under otherwise constant electrical sputtering parameters. This rate is a measure of the basic stability of the reactive process. If the reaction gas flow is less than the critical flow, the magnetron cathode operates in the so-called "metallic range" in which the discharge can be stabilized by very simple means. In this range of operation the required coating processes are generally achieved by keeping such process parameters as power and reaction gas flow constant, and by timing.
On the other hand, in the "metallic range" it is generally impossible to deposit coatings of reaction products with the required stoichiometry. By taking special measures to eliminate the influence of the reaction gas on the target or on the substrate, such coatings can be produced at correspondingly lower rates (see, for example, the previously mentioned DE-OS 33 31 707).
In the procedures described above for the attainment of the required coating properties, the reaction gas flow and/or an electrical working parameter have been either kept constant or controlled.
In controlling the process on the basis of a measured property of the coating, however, not only is the previously mentioned time interval between the end of the coating and the measurement disadvantageous, but also the time lag in the covering of the target is even more critical. Necessary changes in the covering of the target, which occur for example in the case of a change in the power, can bring about a time lag that is greater than the above-mentioned time interval resulting from the location of the point of measurement of the coating parameters. This time shift can amount to between several seconds and several minutes, so that unacceptable changes in the coating process are the consequence.
For economical reasons, coatings generally can be achieved with a high reactivity of the compound and high rates of deposition only in the so-called "transitional mode," which is an unstable or metastable state of operation of the sputtering process, which has been referred to above. In the erosion pit in the target, coating with reaction products is just barely prevented, while the reaction products are deposited on the substrate with a high reactivity and at high rates. Working in the "transitional mode" requires the stabilization of the discharge by additional means.
U.S. Pat. No. 4,166,784 discloses a method of regulation by detecting the intensity of a characteristic line of the target material in the plasma. The stabilization of a working point of the reactive discharge is performed by means of a control circuit by which an electrical parameter of the power supply at constant reaction gas flow is regulated on the basis of the intensity signal as the found value, or the reaction gas flow is controlled at constant electrical operating parameters.
Through DD-PS 239,810 it is known to tune light radiation out of a plasma discharge and use a spectral line or group of spectral lines to form a signal for the purpose of regulating the gas flow. DD-PS 239,811 also discloses how a reference signal can be found for fixing the position of the working point.
control circuits for assuring the process stability of a given working point of the discharge in a reactive cathode sputtering process utilizing an emission signal generally cannot prevent the properties of the coating from being out of tolerance and/or from shifting over a long period of time. There are a number of reasons for this:
When new substrate surfaces are brought into the coating area, surface coatings are released by the action of the plasma on the substrate. This causes the gas composition in the discharge, and consequently also the emission signal, to vary in an uncontrollable manner. PA0 Desorption from parts of the apparatus after a target change likewise produces undesired changes in the emission signals. What is critical in this phenomenon is its uncontrollability and the fact that it is present over long periods of time, even for hours. PA0 Furthermore, all geometrical changes during the coating process, especially progressive target erosion, produce unwanted changes in the emission signal. PA0 (a) An optical sensor of a spectral photometer system is aimed at the space between the magnetron cathode and the substrate, the sensor beam path being parallel to the target surface and being disposed very close to the latter, PA0 (b) A gas distributing system is disposed close to the target surface and is connected through a control valve to a source of the reaction gas, PA0 (c) The output of the spectral photometer system is connected through an amplifier and a controller to an actuator of the control valve, the optical sensor, spectral photometer system, amplifier, controller, actuator and control valve forming a first control circuit with a short time constant, equal to or less than 150 ms and preferably equal to or less than 50 ms, PA0 (d) In the path of the substrate coated immediately before, a second sensor for sensing at least one property of the coated substrate is disposed, the output of the second sensor being delivered to a second amplifier whose output is delivered to a reference value generator for the controller, while the second sensor, the second amplifier, the reference value generator, the controller, the actuator and the control valve connected to the output form a second control circuit with a longer time constant than that of the first control circuit, and the first and second control circuits are connected together in the controller.
The problem to which the invention is addressed is to devise a process of the kind described above, whereby it will be possible in a reactive coating process to regulate the coating properties and achieve close tolerances.